As a conventional image processing method of this type, a binarization method based on error diffusion is generally adopted which can express substantially both sharpness and tone for an image including a halftone image and a thin character line. If the print dot density is raised to 600 DPI or more, the character sharpness improves and the dot graininess of the halftone part is reduced. If the printing density is reduced to 400 DPI, high-quality tone reproduction can be obtained by continuous pulsewidth modulation of 256 gray levels.
In some cases, a printer for printing characters divides one dot of 600 DPI into two dots in the main scanning direction, i.e., prints minimum print dots at 1,200×600 DPI. At this time, correction dots for smoothening a curve part are generated from font data of 600×600 DPI to enable smoother printing at higher resolution.
If ternary processing is performed for each pixel at a printing density of, e.g., 600×600 DPI using similar print dots and multivalued error diffusion, printing by 7-valued pulsewidth modulation is locally realized at a printing density of 200 DPI. This provides an image corresponding to continuous pulsewidth modulation of 256 gray levels at about 200 DPI.
However, compared to binary printing at 600 DPI, the amount of information required for printing using 256 gray levels for 8-bit pixels at a printing density 400 DPI increases by:(400×400×8)/(600×600×1)=32/9=3.56 timesThe amount of information required for printing at 1,200×600 DPI obtained by dividing one dot of 600 DPI into two increases by:(1,200×600×1)/(600×600×1)=2 timesIn digital PWM (Pulsewidth Modulation) printing using ternary error diffusion processing for respective pixels of 600×600 DPI, the number of printing patterns as processing results is 33=27 (5 bits) at a printing density of 200×600 DPI. This amount of information corresponds to 600×600×1.67 bits, which is 1.67 times the amount of information of 600×600 DPI. The increase in the amount of information causes an increase in the cost for storing/transmitting information.